| Document and Entity Information Document and Entity Information | Sep. 17, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 17, 2019 |
| Entity Registrant Name | THE CONNECTICUT LIGHT AND POWER COMPANY |
| Entity Central Index Key | 0000023426 |
| Entity Incorporation, State or Country Code | CT |
| Entity File Number | 0-00404 |
| Entity Tax Identification Number | 06-0303850 |
| Entity Address, Address Line One | 107 Selden Street |
| Entity Address, City or Town | Berlin |
| Entity Address, State or Province | CT |
| Entity Address, Postal Zip Code | 06037-1616 |
| City Area Code | 800 |
| Local Phone Number | 286-5000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
